Discrimination of Ca(H) and Mg(H) Ions Using 2-Acetyl-8-hydroxyquinoline-5-sulfonic Acids

Abstract

Sodium 2-acetyl-8-hydroxyquinoline-5-sulfonate(1) and sodium 2,4-diacetyl-8-hydroxyquinoline-5-sulfonate(2) were synthesized and characterized. The acid dissociation constant of 1 and 2, and the complex formation with Ca (II) and Mg (II) ions were determined from UV measurements in aq. solution at 25 °C: the pKa of the phenolic OH (ionic strength: 0.1) were 7.81 (1) and 7.45 (2), and pKa of the ring-N (Hammett's acidity function scale) were —0.31(1) and —1.40(2). The logarithm of formation constants of the metal complexes (the ratio 1 :1) (ionic strength : 0.1) was 3.87 (1-Ca), 1.48 (1-Mg), 3.60 (2-Ca), and 1.42 (2-Mg). The absorption maximum of the Ca complexes with 1 and 2 shifted to a longer wave-length, while there was no change in that of the Mg complexes. It is suggested that there is no chelating structure in the Mg complexes with 1 and 2 which act as weak monodentate ligands toward Mg in aq. solution. © 1998, The Chemical Society of Japan. All rights reserved.